Improvement of UV-Stability of Porphyrin-Type Humidity Indicator by the Addition of UV-Absorbents

Fukuda Koji, Sugano Kento, Matsumoto Tomoko, Matsumoto Jin, Yasuda Masahide, Fueda Yoshiyuki, Shiomori Kouichiro

doi:10.1252/kakoronbunshu.43.123

<p>A cobalt-free humidity indicator (HDI) for silica gel was developed using tetraphenylporphyrin (Tpp), which showed a distinguishable color change depending on pH. The HDI was prepared by mixing dichloro(tetraphenylporphyrinato)phosphorus chloride (PTpp) and MgCl₂ with SiO₂ and drying at 130°C for 24 h. During the preparation, the PTpp was decomposed into the protonated Tpp (H₂Tpp²⁺). The pH change arose from proton release by the reaction of MgCl₂ with silanol of the SiO₂ under dry conditions and neutralization under humid conditions. The HDI showed green color due to H₂Tpp²⁺ under dry conditions and orange color due to Tpp under humid conditions. However, the HDI underwent partial decoloration on extended exposure to sunlight. Here, to prevent this decoloration, UV-absorbents coumarin and dibenzosuberenone were loaded onto the HDI. The HDIs with coumarin and without coumarin were irradiated at 352 nm and their microscopic absorption spectra were measured on a confocal laser scanning microscope. The absorbance at 650 nm and 515 nm of the HDI were monitored under dry and wet conditions, respectively. The light-protecting ability of UV-absorbent was analyzed by assuming that the decomposition process obeyed first-order kinetics. It was concluded that the additions of 0.125 wt% of coumarin and 0.050 wt% of dibenzosuberenone were effective to prevent decoloration of green color of H₂Tpp²⁺ as well as orange color of Tpp.</p>

Improvement of UV-Stability of Porphyrin-Type Humidity Indicator by the Addition of UV-Absorbents

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